In recent years, with increasingly high performance in various mechanical apparatuses, usage environments of mechanical parts or apparatuses for which a rolling fatigue life is required have become severe. Thus, a demand for improvements in the operating life and reliability of these mechanical parts or apparatuses is increased. In response to such a demand, as a measure in terms of steel materials, there has been conducted proper adjustment of steel ingredients or reduction of impurity elements detrimental to a rolling fatigue life, to improve the operating life and the reliability.
Among impurity elements contained in a steel composition, for example, oxygen is an element composing an oxide-based inclusion, such as alumina, which may originate a failure. Accordingly, the content of oxygen with a particularly high detrimentalness has been reduced to ppm order. The content of oxygen may be further reduced by special melting such as VAR or ESR when further high quality is demanded. Further, measures have been taken to prevent the adverse effects of other impurity elements by reducing the contents of the elements to 0.01 mass % order.
There has been variously proposed high cleanliness steel with a low oxygen content in the steel. Among these proposals, a high-carbon-based long-life bearing steel having a value of {(the number of MgO.Al2O3+the number of MgO)/the number of all oxide-based inclusions} of 0.80 or more has been proposed in terms of the number of oxides in the steel (for example, see Patent Literature 1). In Patent Literature 1, the composition range of MgO and Al2O3 is not particularly described. Because of the expression not by MgO—Al2O3 but by MgO.Al2O3 showing a stoichiometric composition in a molecular formula, a compound comprising, in mass %, 28.3% of MgO and 71.7% of Al2O3 is expressed. Furthermore, there have been proposed a high-carbon chromium bearing steel in which the total number of alumina-based oxides and spinel-based oxides is less than 60% of the total number of oxides, and a method for producing the steel (for example, see Patent Literature 2). As far as this Patent Literature 2 is concerned, it is defined that the alumina-based oxide is an oxide in which each of (MgO) and (SiO2) is less than 3% and the ratio of (CaO)/((CaO)+(Al2O3)) is 0.08 or less in terms of (CaO), and the spinel-based oxide is an oxide having a spinel type crystal structure in which 15% or less (CaO) and/or 15% or less (SiO2) may be mixed into a binary oxide including (MgO) in a range of 3% to 20% with the balance of (Al2O3). Furthermore, there has been proposed a high cleanliness bearing steel in which an oxygen content in the steel is less than 10 ppm and the surface-exposed area of oxide-based inclusions floated and aggregated by an electron beam melting method is 20 μm2 or less per gram (for example, see Patent Literature 3). On the other hand, in the case of stably providing a steel having an excellent rolling fatigue life, targeted by the present invention, i.e., a steel having an excellent L1 life (cycle number at which 99% of test pieces rotate without peeling when the test pieces of which the lots are the same are tested on the same condition) in a thrust type rolling fatigue test, non-metallic inclusions of more than 20 μm, influencing an L1 life, are extremely accidentally and less probably generated. Therefore, it is very difficult to detect the generation of the non-metallic inclusions. Moreover, in the steel described in Patent Literature 3, inclusions are melted and aggregated, and therefore, it is likely impossible to accurately evaluate the diameters and number of inclusions. Further, in a method for evaluating non-metallic inclusions according to conventional art, examination of the large volume of a steel material requires a great deal of time due to a small area to be detected. Therefore, it is difficult to judge whether the steel material is good or poor.
Further, there has been proposed an evaluation method using, in combination, both techniques of statistics of extreme values and ultrasonic flaw detection, in which, e.g., the statistics of extreme values is applied to inclusions having a maximum inclusion diameter of approximately 100 μm or less and the ultrasonic flaw detection at a flaw detection frequency of 5 to 25 MHz is applied to inclusions of approximately 100 μm or more (for example, see Patent Literature 4). This literature proposes the evaluation method by the combination in which, e.g., the statistics of extreme values is applied to non-metallic inclusions having a maximum inclusion diameter of less than 100 μm and the ultrasonic flaw detection at a flaw detection frequency of 5 to 25 MHz is applied to non-metallic inclusions of 100 μm or more. However, in the statistics of extreme values, an area to be detected is small similarly as described above, it is possible to insufficiently judge whether a steel material is good or poor in terms of non-metallic inclusions of 20 μm or more and less than 100 μm. On the other hand, because the diameters of inclusions detected by the ultrasonic flaw detection at a flaw detection frequency of 5 to 25 MHz are 100 μm or more, it is still possible to insufficiently evaluate inclusions of 20 μm or more and less than 100 μm. Therefore, an evaluation method that enables steel having an excellent L1 life to be stably provided is demanded. Further, there has been proposed steel in which the number and sizes of inclusions are set for the steel having an excellent rolling fatigue life by evaluating inclusions of 100 μm or less by ultrasonic flaw detection at a flaw detection frequency of 20 to 125 MHz (for example, see Patent Literature 5). In a method described in this Patent Literature 5, there have been proposed the steel having an excellent rolling fatigue life wherein the number of non-metallic inclusions which have a sulfur content of 0.008 mass % or less and in which the diameters of inclusions detected by ultrasonic flaw detection are 20 μm or more per steel material volume of 300 mm3 is set to be 12 or less per 300 mm3 (steel in which L10 life>1.0×107 cycles is obtained at a maximum Hertzian stress Pmax=5.3 GPa in a thrust type rolling fatigue test). Also, there have been proposed an evaluation method of the steel. However, in the method, reliability against the failure of a bearing being used, occurring much earlier than a calculated life, is not evaluated, and therefore, it is possible to fail in stably providing steel having an excellent L1 life (cycle number at which 99% of test pieces rotate without peeling when the test pieces of which the lots are the same are tested on the same condition) which is an index for the reliability against early failure.